A so-called local area network has recently been developed in which a plurality of microcomputers and terminal units are connected to each other by a communication line or channel so that the network can be efficiently utilized for communications between microcomputers and the terminal units.
Various kinds of local area networks are introduced and explained, for example, in a magazine entitled "Nikkei Electronics" pp. 185-238 and published on Apr. 22, 1985. As one of the local area networks, there is described a communication system in which a main station including a computer is connected by a loop communication line with a plurality of various I/O systems having terminal units connected thereto so as to transmit and receive data between the main station and the individual I/O systems.
Further, various kinds of local area network systems are explained in a book entitled "Microcomputer Local Network" pp. 11-31, written by T. Mizuno and published by Sanpo Shuppan K.K.
In most of the communication systems described above, a destination code is added to transmitted data to specify the destination of the transmitted data, and, when so required, information including the total number of bits of the transmitted data and an end code are also added. That is, the destination code is detected to determine the data receiving station, and the total number of bits is used to determine the amount of transmitted data. In this case, it is necessary to provide means for judging whether the communication line or channel is presently idle and available or not. Further, the data transmitting station must have a capability of computing the destination address and the total number of bits of transmitted data and sending out the data to the communication channel according to the communication data format. The data receiving station must have a capability of receiving all the data transmitted thereto by way of the communication channel and determining whether the data is directed to that station or not and a capability of informing the data transmitting station of the completion of reception of the data when it has received the data directed to that station.
Because of the capability requirements described above, the main station and the I/O systems of the terminal stations require complex structures, and the software required for controlling the operations of the stations is complex, resulting in relatively low reliability of the communication system. Also, the cost of the communication system inevitably become high. Although such a communication system is preferable for transmission of a large amount of data at one time, the data transmission efficiency is not high when the amount of data to be transmitted at one time is not large. Further, since the transmitted data includes address data, etc., other than the necessary information, an extended period of time is required for the transmission and reception of the data. When, for example, a pulse motor in one of the terminal stations is to be controlled by an instruction signal transmitted from the main station, several bits of data are transmitted from the main station to control the pulse current supplied to the field coil of the pulse motor. In the case of such control, it is necessary to transmit the control data to the terminal equipment at an interval shorter than the period for controlling the rotation of the pulse motor. Also, when the main station produces the control signal on the basis of an operating position detection signal transmitted from the terminal station, the position detection signal must be transmitted from the terminal station to the main station at a high speed.